Convert java.io.Reader to the other Reader with skip reading some chars

Question

I have java.io.Reader as input parameter to the method.

This reader reads file, like "abc\def\jhk"; I need to return from the method an other java.io.Reader, which skips reading "\" chars. So the final file should be: "abcdefjhk".

private Reader convertToFilteredReader(Reader reader) {
    Reader filteredReader = null;
    // some code here
    return filteredReader;
}

I have made this task by converting Reader to String, calling String.replace('\', '') and then converting this String into Reader. But I don't like this solution because of double converting (Reader -> String -> Reader).

Answer 1

Give this a try. Disclaimer: only superficially tested. Comments show how this is simple to reuse.

import java.io.StringReader;
import java.io.Writer;
import java.nio.file.Files;
import java.nio.file.Path;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.FilterReader;
import java.io.IOException;
import java.io.Reader;

/**
 * Subclasses should implement isExcluded to determine which characters will be
 * 'swallowed' by the Reader
 *
 * @author g00se
 * @version 1.0
 */
public abstract class ExclusionReader extends FilterReader {
    private char[] singleCharBuf = { 0 };

    public ExclusionReader(Reader in) {
        super(in);
    }

    // Testing only
    public static void main(String[] args) throws IOException {
        Reader downstream = args.length < 1 ? new StringReader("abc\\def\\ghi")
                : Files.newBufferedReader(Path.of(args[0]));
        try (Reader in = new BackslashExcludingReader(downstream);
                Writer out = new OutputStreamWriter(System.out)) {
            in.transferTo(out);
        }
    }

    public abstract boolean isExcluded(int codePoint);

    @Override
    public int read() throws IOException {
        int result = read(singleCharBuf, 0, 1);
        return result > -1 ? singleCharBuf[0] : result;
    }

    @Override
    public int read(char[] b) throws IOException {
        return read(b, 0, b.length);
    }

    @Override
    public int read(char[] buf, int off, int len) throws IOException {
        if (len <= 0) {
            if (len < 0) {
                throw new IndexOutOfBoundsException();
            } else if ((off < 0) || (off > buf.length)) {
                throw new IndexOutOfBoundsException();
            }
            return 0;
        }
        int charsCopied = 0;
        int c = -1;

        while ((charsCopied + off) < len && (c = in.read()) > -1) {
            if (!isExcluded(c)) {
                buf[off + charsCopied] = (char) c;
                charsCopied++;
            }
        }

        int returnVal = charsCopied;
        if (charsCopied == 0) {
            returnVal = Math.min(charsCopied, c);
        }
        return returnVal;
    }

    // What YOU need
    static class BackslashExcludingReader extends ExclusionReader {
        public BackslashExcludingReader(Reader in) {
            super(in);
        }

        @Override
        public boolean isExcluded(int codePoint) {
            return codePoint == '\\';
        }

    }
}